Catalytic incineration (oxidation) is an energy efficient method of purifying waste gases, e.g. vapor of solvents, inks, paints, etc. which contain noxious and/or toxic organic components including hydrocarbons and oxygenated hydrocarbons such as alcohols, esters, acids, etc. Such a process involves contacting the waste gas stream with a catalyst in the presence of excess oxygen at a temperature below about 600.degree. C. The contact or residence time of the waste gas with the catalyst is very short, on the order of less than 0.1 seconds. However, the presence of halogenated compounds in the gas stream usually prohibits the use of this process because the catalysts which are used are poisoned or deactivated by the halogen compounds.
Streams containing halogenated organic compounds, referred to hereinafter as organohalogen compounds, usually must be purified by thermal incineration at temperatures of at least 1100.degree. C., using reactors which are large enough to provide long residence times, on the order of greater than 1 second. Thus, thermal incinerators have two disadvantages; 1) the gas stream must be heated to high temperatures, requiring consumption of large amounts of fuel, and 2) the large reactors require a large capital investment. Therefore, there is a need for a catalyst which can destroy organohalogen compounds at lower temperatures and shorter residence times.
The prior art shows that gas streams containing simple organohalides can be oxidized using a catalyst. For example, U.S. Pat. No. 4,039,623 teaches that a waste gas containing C.sub.2 -C.sub.4 halogenated hydrocarbons may be treated by contacting the waste gas with a hydrated nickel oxide catalyst. The process described in the '623 patent works best on unsaturated chlorinated hydrocarbons such as vinyl chloride.
U.S. Pat. Nos. 4,059,675, 4,059,676 and 4,059,683 respectively disclose the use of catalysts containing ruthenium, ruthenium plus platinum and platinum dispersed on a non-oxidizing support to decompose chlorinated organic compounds having one to four carbons. The halogenated organic compounds are characterized in that the total number of hydrogen atoms is at least equal to the total number of halogen atoms in the compound. The compounds are contacted with the catalyst at a temperature of at least 350.degree. C. The oxidation products are CO.sub.2, H.sub.2 O, HCl and Cl.sub.2. Thus, both HCl and Cl.sub.2 are produced using these catalysts and processes. The production of Cl.sub.2 is undesirable because it is extremely corrosive.
Japanese Disclosure J61141919-A teaches that an exhaust gas containing 1,1,1-trichloroethane can be treated by contacting with one or more catalytic oxides of vanadium, chromium, tungsten, manganese, cobalt and nickel. The gas must be contacted with the catalyst for 1-30 seconds at 150.degree.-300.degree. C. Finally, Murakami et al. (see Preprints of Papers of The Seventh International Congress on Catalysis, Jul. 3-4, 1980, Tokyo, Japan, paper B49) disclose that a vanadium oxide on titania catalyst can oxidize benzene, but of the benzene which is oxidized only half of the benzene is completely oxidized, while the other half is converted to maleic anhydride.
It has also been reported by G. C. Bond and N. Sadeghi in J. Appl. Chem. Biotechnol., 25, 241-248 (1975) that a Pt on gamma alumina catalyst could be used to destroy compounds such as CCl.sub.4, CHCl.sub.3, etc. However, their method requires the combustion of a hydrocarbon fuel and must be run at temperatures above 420.degree. C. in order to be effective. Finally, a review of the state of the art has been published by J. J. Spivey, Ind. Eng. Chem. Res., 26, 2165-80 (1987).
What these references indicate is that a process is not available which can convert organohalogen compounds and especially C.sub.1 organohalogen compounds that do not contain any C-H bonds, e.g., CCl.sub.4, ClCOOH, CF.sub.2 Cl.sub.2, CF.sub.4, etc., to carbon dioxide, water, and haloacids (haloacids are HCl, HBr, etc.) at a low temperature and high space velocity. These conditions must be met if a process is to have commercial success. Applicant has addressed this problem and has discovered catalysts which can be tailored to the compounds present in the gas stream such that the gas stream can be effectively treated at temperatures as low as 300.degree. C. and a space velocity of about 15,000 hr.sup.-1.
Applicant's process involves contacting a gas stream with a catalyst at operating conditions. If only C.sub.1 organohalogen compounds that do not contain any C-H bonds need to be converted, then the catalyst contains titania and optionally tungsten oxide. If both organohalogen compounds and other organic compounds need to be converted a titania catalyst may be used, although a preferred catalyst will contain titania and vanadium oxide and optionally tungsten oxide and/or a noble metal such as platinum. Finally, tin oxide may be added to any of the above catalysts primarily to increase their stability. Using applicant's catalyst, 99% of the carbon tetrachloride in a gas stream is converted to carbon dioxide and hydrogen chloride at a temperature as low as 240.degree. C. and practical residence times of less than 0.3 seconds. Applicant is the first person to accomplish such a result.
A comparison of the instant catalyst to those reported in the prior art quickly reveals the tremendous advantages of the instant catalyst. For example, applicant's catalyst completely oxidizes benzene to CO.sub.2 and water whereas the catalyst of the prior art (Murakami) oxidizes benzene to CO.sub.2 and water and to maleic anhydride. Obviously the latter is an undesirable result if one wishes to treat a gas stream containing such chemicals. Additionally, the catalysts of U.S. Pat. Nos. 4,059,675, 4,059,676, 4,059,683 and 4,039,623 are claimed to be effective for oxidizing only those halogenated compounds containing 1 to 4 carbon atoms (excluding carbon tetrachloride). In contrast, applicant's catalyst is able to convert a variety of organohalogen compounds to innocuous compounds regardless of the number of carbon atoms or whether hydrogen atoms are present on the compound or not. Even the Bond and Sadeghi reference which reports conversion of carbon tetrachloride requires temperatures of at least 420.degree. C. and the presence of a hydrocarbon fuel. Therefore, applicant's process represents a substantial improvement in the art.